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KEF's Breakthrough VECO Speaker Technology to Lower Distortion–Explained by Its Inventors

olieb said:
this is not exactly what you want for a coax driver ;-)
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I agree (also regarding tinkering looks but prescribe this to early production in the 1970s), but render a sensor-controlled midwoofer diaphragm for a conventional coaxial speaker largely useless. Motional negative feedback loops with electrodynamic sensors are mostly useful in the (low) bass region with a closed-box design woofer arrangement, lots of excursion, stiff diaphragm and power. So to say in a situation in which the woofer diaphragm is prone to simply losing it. All the ingredients you do not want in a KEF-style coax as you want to keep midwoofer´s excursion and its influence on the treble as low as possible.

Frank Dernie said:
helping a 2-way uni-q.
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As mentioned, this is rather unlikely, because in a closed-box design you would drive up midrange diaphragm excursion up to a level which is not suitable for a 2-way coaxial design. In a vented design, on the other hand, the main point of sensor controlling is lost.
 
Interesting, but seems more like marketing rather than a marked improvement.
 
Arindal said:
As mentioned, this is rather unlikely, because in a closed-box design you would drive up midrange diaphragm excursion up to a level which is not suitable for a 2-way coaxial design. In a vented design, on the other hand, the main point of sensor controlling is lost.
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Maybe do hold your breath for veco in 2-way. Volume displacement is limited, so we can't use it to extend the bass, but there are still benefits. Not least that we can use a current output amplifier which avoids power compression in the midrange and gains a small amount of inductance related distortion reduction. In the low frequency we get a touch less distortion and we can fine tune the system qts to achieve different vented alignments than with the driver open loop.
 
mhardy6647 said:
I didn't think that's what breakthrough meant -- guess I'll check my dictionary later today! ;)
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The video title wasn't chosen by us. The trouble is that speaker engineering is an 80 year mature topic. Nothing is totally new and we're not trying diminish the achievements of anyone whose already implemented servo control. The main thing is the new sensor approach allowing us to apply it in compact assemblies and across multiple products. I've been very busy with the product development, but do plan to write a white paper with some more technical details (relatively) soon.
 
Petrushka said:
Seems like it would be simpler to use something like a bar code reader to detect cone motion. Could be retrofitted to existing drivers,
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Guarantee the laser diode would never be out of alignment inside a speaker box… lol
 
jackocleebrown said:
… different vented alignments than with the driver open loop.
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Sorry, didn‘t dig into it yet, but would that application to ‚vented’ be model driven? Just curious what I may have overlooked. Otherwise, looking forward to your paper. KEF‘s have always been a delight, since the 70s actually, thanks!
 
ppataki said:
Sorry for my ignorance but Kef just 're-invented' the active servo processing? Or is this something different?
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I think the difference here is with modern DSP processors being way more powerful than anything that was available in the past, making it viable to actually do on-line real-time speaker linearization. The approximations due to tech limitations in past attempts indeed lead to quite modest results, as everybody is pointing out.

The other important point is that indeed the voice coil pickup is averaging the non-linearity measurements. Previous tech based on measuring the movement of a single point of the speaker surface were subject to measuring local speaker vibration modes rather than the speaker's suspension non linearities.

I think it is pretty cool, especially in small speaker systems that work way off the cone/suspension linearity zone.
 
Even if its evolution of existing technology rather than groundbreaking invention, I still applaud KEF for their continued scientific endeavours. They have a track record of making their research efforts into commercially viable solutions.
 
fricc said:
I think the difference here is with modern DSP processors being way more powerful than anything that was available in the past, making it viable to actually do on-line real-time speaker linearization.
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Thats the key. Delays limit feedback bandwidth. DSP always adds delay. At 10khz a 50us delay will cause instability.

Im waiting for the measurements.
 
Design is old but servo processing has gotten better and magnets have much more strength which leads to better precision. Looking forward to seeing the w/wo graphs. Did not view video yet.
 
Heinrich said:
Sorry, didn‘t dig into it yet, but would that application to ‚vented’ be model driven? Just curious what I may have overlooked. Otherwise, looking forward to your paper. KEF‘s have always been a delight, since the 70s actually, thanks!
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No models involved. With a current output amplifier (high output impedance) the Qts becomes equal to Qms because the motional emf has no effect on the current. If negative velocity feedback is added around the outside of this the effect is the same as adding damping back into the system, so it can effectively be used to reduce the Qts back down to something appropriate for a vented box. The designer is free to choose as much or little velocity feedback as they like. Probably less feedback will be applied in this scenario than a closed box because beyond a certain point the port begins to get uncontrolled. However, it's still a useful amount and better than a conventional vented system because the motional emf is nonlinear (due primarily to BL variation with voice coil position) whereas the sensor output is pretty linear. Second, you get the benefits of having a current output amp.
 
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Cbdb2 said:
Thats the key. Delays limit feedback bandwidth. DSP always adds delay. At 10khz a 50us delay will cause instability.

Im waiting for the measurements.
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They only linearize the low frequency response, where the suspension is non-linear. At higher frequencies modern speakers usually have very little distortion.
 
mhardy6647 said:
Yeah, Philips was kind of a mom & pop store in the 1970s, weren't they? ;)



There's been a lot of breakthrough inflation in the past 3 or 4 decades.
:rolleyes:
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The powered Philips MFB RH544 with motional feedback (and other interesting features) was used in a few recording studios at the time and was apparently used in the mixing of at least one Pink Floyd album.
 

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Can we quantify this? If a good speaker has 1% distortion at 60 Hz at 90 db SPL, how much would VECO reduce that?
It would also be valuable to quantify this psychoacoustically. If VECO is used for low frequencies, that's where distortion is harder to hear, so what improvement does it make?
 
Cbdb2 said:
You also get the disadvantages.
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Essentially none of the disadvantages listed in that article apply here.
Severely underdamped bass? Solved by velocity feedback.
Bad interaction with passive crossovers designed for voltage drive? Clearly not an issue here since there's no passive crossover.
Amplifier instability with arbitrary loads? The driver, amp, and velocity feedback loop are a tightly integrated system and aren't intended to be mucked with by the end user.
Power compression as a means of protecting the driver? For a system like this, much better results can surely be had by incorporating even a basic thermal model and limiting mechanism.

Besides the additional complexity and (probably modest) cost of the sensor, I see only upsides in this application.
 
MRC01 said:
Can we quantify this? If a good speaker has 1% distortion at 60 Hz at 90 db SPL, how much would VECO reduce that?
It would also be valuable to quantify this psychoacoustically. If VECO is used for low frequencies, that's where distortion is harder to hear, so what improvement does it make?
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It depends on the driver. The distortion reduction comes from two parts. First from the current output amp. If you have a very simple motor without any special features for controlling inductance distortion then this alone could be a decent 15dB reduction (see below) and in relatively audible frequency ranges. The second is from the velocity feedback itself and that depends on how much feedback is applied, and the type of system design.

I am planning on writing an AES paper with some examples. Here's one from an early prototype with quite a crude VECO design. This is a very basic 10inch LF with a simple motor and four layer coil. Most of the distortion reduction here was from the current drive amp.

1000028377.jpg
 
I'm surprised that everyone focused just at the servo control and lefr current drive out of the equation.
I wrote it at the 3rd post of the thread, no one has ever read about it?

Ancient active systems (each driver with its own tube amp or high output impedance amp) were optimized that way, essentially tailored and working as a system.
 
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